Creation of Single Chain of Nanoscale Skyrmion Bubbles withRecord-High Temperature Stability in a Geometrically ConfinedNanostripeZhipeng Hou,†,‡ Qiang Zhang,‡ Guizhou Xu,§ Chen Gong,‡ Bei Ding,† Yue Wang,† Hang Li,† Enke Liu,†

Feng Xu,§ Hongwei Zhang,† Yuan Yao,† Guangheng Wu,† Xi-xiang Zhang,*,‡ and Wenhong Wang*,†

†Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190,China‡Physical Science and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, SaudiArabia§School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China

*S Supporting Information

ABSTRACT: Nanoscale topologically nontrivial spin textures, such as magneticskyrmions, have been identified as promising candidates for the transport andstorage of information for spintronic applications, notably magnetic racetrackmemory devices. The design and realization of a single skyrmion chain at roomtemperature (RT) and above in the low-dimensional nanostructures are of greatimportance for future practical applications. Here, we report the creation of asingle skyrmion bubble chain in a geometrically confined Fe3Sn2 nanostripe with awidth comparable to the featured size of a skyrmion bubble. Systematicinvestigations on the thermal stability have revealed that the single chain ofskyrmion bubbles can keep stable at temperatures varying from RT up to a record-high temperature of 630 K. This extremestability can be ascribed to the weak temperature-dependent magnetic anisotropy and the formation of edge states at theboundaries of the nanostripes. The realization of the highly stable skyrmion bubble chain in a geometrically confinednanostructure is a very important step toward the application of skyrmion-based spintronic devices.

KEYWORDS: Skyrmion bubble, temperature stability, frustrated magnet, Lorentz transmission electron microscopy

Magnetic skyrmions are topologically protected vortex-likespin textures that were theoretically predicted two

decades ago.1 Recently, they have been experimentally observedin a small class of noncentrosymmetric magnets with a helicalmagnetic ground state caused by the Dzyaloshinskii−Moriyainteraction (DMI).2−8 Since the first experimental confirmationmade by Muhlbauer et al.2 with neutron scattering andsubsequent confirmation of ultralow threshold for current-driven motion (∼105−106 A m−2)9 in the chiral magnet MnSi,magnetic skyrmions have been considered as promising carriersof information for spintronic applications,10−16 particularly theracetrack memory devices.11,17 However, the overall lowthermal stability of magnetic skyrmions has been one of themajor factors hindering their practical applications.2,4−7 Recentexperimental studies have demonstrated that their temperaturewindow can be expanded significantly by means of zero-fieldcooling (ZFC),18,19 field cooling (FC),20−24 lowering thedimensionality,3,25−31 electric field,32 or pressure.33 Remark-ably, Du et al.31 have realized a single skyrmion chain in ageometrically confined FeGe nanostripe over a wide temper-ature range from 100 to 250 K, which makes significantprogress toward the fabrication of skyrmion-based racetrackmemory devices. However, for practical applications, the singleskyrmion chain requires to be created and manipulated not

only within a wide temperature range, but also at roomtemperature (RT) and even well above.Alternatively, several groups have recently reported the

observation of so-called magnetic skyrmion bubbles in thecentrosymmetric magnets, such as Sc-doped hexagonal bariumferrite,34 bilayered magnetite,35 Ni2MnGa,36 MnNiGa,37

La1−xSrxMnO3 (x = 0.175),38 and the frustrated magnetFe3Sn2,

39 in which the skyrmion bubbles are stabilized by acompetition between the magnetic dipole interaction anduniaxial anisotropy. Skyrmions and skyrmion bubbles aretopologically equivalent to each other and possess somecommon topological properties, such as the topological Halleffect,37,40 skyrmion Hall effect,41,42 and ultralow currentdensity that controls their motion.9,35 Most importantly, thematerials with skyrmion bubbles usually exhibit a highermagnetic ordering temperature, and the exchange interaction isgenerally stronger than the DMI.43 These features makeskyrmion bubbles able to be stabilized, not only over a muchwider temperature range but also at much higher temperatures.

Received: November 20, 2017Revised: December 21, 2017Published: January 4, 2018

Letter

pubs.acs.org/NanoLettCite This: Nano Lett. 2018, 18, 1274−1279

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Moreover, recent theoretical simulations have shown that theformation and stability of individual skyrmion bubbles in thenanostructures of frustrated magnets are strongly affected bythe edge states at the boundaries.42 Therefore, experimentallycreating a single chain of skyrmion bubbles in thenanostructures and further investigating their temperaturestability are pivotal for both the applications of skyrmion-based racetrack memory and the fundamental insight into thedynamical mechanism of skyrmion bubbles.Here, we report the design and experimental fabrication of

nanostripes with well-defined geometries based on thefrustrated magnet Fe3Sn2 (detailed in the SupportingInformation, SI, Note 1). Intriguingly, a single chain ofskyrmion bubbles was created in the Fe3Sn2 nanostripe witha width comparable to the featured size of the bubbles. Furthersystematic investigations on the thermal stability showed thatthe single chain could keep stable at elevated temperatures, ashigh as 630 K. To elucidate the mechanisms behind the highstability, we investigated the morphology of the skyrmionbubbles and the evolution of their spin texture under differentexternal magnetic fields, by using high-resolution Lorentz TEM.It was demonstrated that the record-high thermal stability couldbe attributed to the weak temperature-dependent magneticanisotropy of Fe3Sn2 and the edge states resulting from thegeometrically confined effects.Fe3Sn2 is a rare example of the frustrated ferromagnetic

compounds exhibiting a high Curie temperature Tc up to 640 K(see Figure 1a). It undergoes a spin reorientation during whichthe easy axis rotates gradually from c-axis to ab-plane as thetemperature decreases from 640 to 100 K.44−47 This compound

has a layered rhombohedral structure with alternate stacking ofthe Sn layer and the Fe−Sn bilayers along the c-axis. The Featoms form bilayers of offset Kagome networks with Sn atomsthroughout the Kagome layers, as well as between the Kagomebilayers.45 In this work, we synthesized the high-quality Fe3Sn2single crystals by a Sn-flux method. (Detailed informationrelating to the growth of the single crystals and theircharacterizations is given in the SI and Figure S1.) Figure 1bshows the temperature-dependent saturation magnetization(Ms) of an Fe3Sn2 single crystal in the temperature range 700−10 K under a magnetic field applied along the c-axis. It isobserved that their values coincided with previous reports46 anddecreased monotonically as the temperature increased to Tc.Moreover, the overall behavior of Ms(T) can be well-describedby the Bloch formula: Ms(T) = Ms(0)(1 − bT3/2), which isrelated to the localization of the magnetism.48

To investigate the influence of geometric confinement on theformation and stability of skyrmion bubbles, two Fe3Sn2nanostripes with different widths were fabricated from a bulksingle crystal by using a focused ion beam (FIB) technique(Figure S2, SI). The scanning electron microscope (SEM)images demonstrate that both the nanostripes are flat anduniform (Figure S3, SI), suggesting a good quality. Onenanostripe is designed to be 4000 nm in width, which is 15times larger than the diameter D ≈ 280 nm of a magneticskyrmion bubble reported in our previous work,39 to representa sample in which the skyrmion bubbles are not geometricallyconfined. The width of the other one is confined to be only 600nm which is only approximately twice the value for D. It isexpected that the formation of the skyrmion bubbles in the 600

Figure 1. Basic characteristics of the bulk Fe3Sn2 single crystal and 4000 nm nanostripe. (a) Temperature dependence of low-field (0.05 T)magnetization for single crystal of Fe3Sn2. The Curie temperature is determined to be approximately 640 K. (b) Temperature dependence ofsaturated magnetization curve with magnetic field applied along the c-axis. The same data are obtained when the field is applied along the ab-plane.The black solid line is the fitting of the Ms(T) to the Bloch equation: Ms(T) = Ms(0)(1 − bT3/2). (c) A typical scanning electron microscopy(STEM) image of the 4000 nm wide nanostripe and selected-area diffraction pattern are shown in the left and right lower panels, respectively. (d)High-resolution scanning transmission electron microscopy image taken on the [001] axis. The left inset shows the alternate arrangement of Fe (blueparticles) and Sn (red particles) atoms into the Kagome lattice, whereas the right inset displays the crystal structure of the Fe3Sn2. (e) Room-temperature under-focused LTEM images taken at a zero field showing the existence of a helical magnetic structure. The inset depicts the sinusoidalchange in the LTEM contrast with distance as marked by the white line in the main panel. (f) Room-temperature under-focused LTEM images takenin a field of 300 mT showing the existence of skyrmion bubbles.

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nm nanostripe could be significantly influenced by thegeometric confinement.The upper panel of Figure 1c shows a typical SEM image of

the 4000 nm nanostripe used in this study. To present itsstructure details more clearly, we have further shown a scanningtransmission electron microscopy (STEM) image of thenanostripe in the left lower panel of Figure 1c. It should benoted here that the outer parts of the nanostripe are coatedwith amorphous carbon (black region) and platinum (grayregion), which are applied to protect the Fe3Sn2 layer duringthe fabricating process and to reduce the interfacial Fresnelfringes in the Lorentz TEM image. Selected-area electrondiffraction (SAED) was performed on the Fe3Sn2 layer todetermine its orientation. The diffraction spots (the right lowerpanel of Figure 1c) showed a perfect 6-fold symmetriccharacteristic, suggesting that the normal direction was alongthe [001] orientation. In Figure 1d, we show the high-resolution scanning transmission electron microscopy imagealong the [001] direction. It is clear to see that Fe (blueparticles) and Sn (red particles) atoms arranged into theKagome lattice in an alternate fashion (as shown in the upper-right inset), which agrees well with the crystal structure ofFe3Sn2 and confirms that the nanostripes are of good quality.Hereafter, we investigated the evolution of magnetic domains

in the 4000 nm nanostripe at room temperature by using aLorentz TEM instrument equipped with a spherical aberrationcorrector for imaging system (Titan G2 60-300, FEI) at anacceleration voltage of 300 kV. Figure 1e shows the under-focused Lorentz TEM image taken at a zero magnetic field. Thesinusoidal variation of the Lorentz TEM contrast suggests thatthe helix period is about 430 nm (see the inset of Figure 1e). Bygradually increasing the magnetic field along the [001]direction from zero, the magnetic stripe domains underwenta complex process and eventually transformed into skyrmionbubbles in a critical field of 300 mT (see Figure 1f). It is foundthat the transforming process, the morphology of the magneticbubbles, and even the value of the critical field (Figure S4, SI)are quite similar to those observed in the Fe3Sn2 thin plate.38

Therefore, we can conclude that the 4000 nm nanostripe canbe considered as a comparison sample in which the geometric

confinement shows little influence on the formation ofmagnetic skyrmion bubbles.Since the Tc of Fe3Sn2 is as high as 640 K, it is natural to

expect that the skyrmion bubbles can survive up to a highertemperature. To obtain Lorentz TEM images in the high-temperature region, a double-tilt heating holder (Model 652,Gatan Inc.) with a smart set hot stage controller (Model 901,Gatan Inc.) were used to raise the specimen temperature from300 to 670 K. In Figure 2a−f, we show the temperature-dependent under-focused Lorentz TEM images of skyrmionbubbles at temperatures ranging from 300 to 573 K under theircorresponding critical magnetic fields (all the magnetic stripestransform into skyrmion bubbles under the magnetic field).The detailed transformation processes of the magnetic stripedomains are presented in Figure S5 in the SupportingInformation. One can notice that the critical field decreasedcorrespondingly with the increase of temperature. Moreintriguingly, both the diameter and density of the magneticbubbles remain almost unchanged, even at a temperature ashigh as 523 K, indicating a high thermal stability of theskyrmion bubbles in the Fe3Sn2 nanostripe. However, when thetemperature increased above 553 K, the density of the skyrmionbubbles decreased substantially, which suggests that the thermalfluctuations start to make the spin structure of skyrmionbubbles unstable. At 573 K, when we increased the appliedmagnetic field, the magnetic stripes vanished directly withoutgoing through the phase of skyrmion bubble (see Figure 2f).In the case of the 600 nm Fe3Sn2 nanostripe, the evolution

process of the magnetic domains involved a gradual trans-formation of the magnetic stripes into individual skyrmionbubbles when the applied magnetic field increased. TypicalLorentz TEM images of this transformation are given in Figure3a−e. By using the transport-of-intensity equation (TIE)analysis, we present the corresponding spin textures of themagnetic domains enclosed by white boxes in Figure 3a,c,d intheir corresponding insets, respectively. At zero magnetic field,we found that a series of nanosized magnetic stripe domainsarranged along the long axis of the nanostripe with an averageperiodicity of 360 nm, which is slightly smaller than that of 430nm in the 4000 nm nanostripe. As the magnetic field increased,

Figure 2. Temperature dependence of magnetic bubbles in the 4000 nm nanostripe. (a−f) Under-focused LTEM images of the magnetic bubbles inthe 4000 nm nanostripe taken at temperatures ranging from 300 to 573 K under their corresponding critical magnetic field. All the photos are takenin the same region of the nanostripe. Notably, when the temperature rises above 473 K, some dendrite impurities are brought by thermal fluctuationand can adhere to the surface of the sample. The scale bar is 500 nm in all the images.

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the magnetic stripes first became narrow, and then swirled intopocket-shaped domains. When the magnetic field increasedabove 120 mT, the pocket-shaped domains became skyrmionbubbles entirely. Once formed, the bubbles tended to movetoward the interior (due to the repulsion with the edge spins)and assembled to form a single chain along the long axis of thenanostripe. By analyzing the spin distributions of the skyrmionbubbles in the 600 nm Fe3Sn2 nanostripe (see the inset ofFigure 3d), they were found to converge, and therefore weproposed their corresponding topological number N to be 1.Those experimental results can be reproduced by a theoreticalsimulation of the evolution of the skyrmion bubbles withincreasing the perpendicular field, as shown in Figure 3g−j,where the sizes of skyrmion bubbles are revealed to graduallydecrease with increasing fields. In addition, some of theskyrmion bubbles display elliptical shapes which are slightlydistorted from the circular ones. According to recent MonteCarlo simulations and Lorentz TEM experiments,36,49,50 it issupposed that the distortion results from a small in-planecomponent of the oblique magnetic field. We have furtherconfirmed this assumption by carrying out the angle-dependentLorentz experiments (Figure S6, SI). It is clearly demonstratedthat the skyrmion bubbles gradually become ellipticallydistorted by titling the sample. Notably, by careful high-resolution TEM analysis (Figure S7, SI), a small strain wasrevealed in the fabricated Fe3Sn2 nanostripe. We propose that itmay also induce the deformations of skyrmion bubbles, as isdemonstrated in the chiral magnet of FeGe.51

We have further checked the thermal stability of the singleskyrmion bubble chain over a wide temperature range from 300K to Tc (640 K). Figure 4a−d shows the under-focused Lorentz

Figure 3. Evolutionary process of magnetic domains under differentout-of-plane magnetic fields in a 600 nm nanostripe at 300 K. (a−e)Under-focused LTEM images under different out-of-plane magneticfields. The domain enclosed by white boxes presents a one-to-onecorrespondence in the stripe−bubble transformation. Inset: thecorresponding magnetization textures obtained from the TIE analysisfor the domains enclosed by white boxes. The scale bar is 500 nm. (f−j) Micromagnetic simulation of the evolution of the single chain ofskyrmion bubbles as a function of the magnetic field.

Figure 4. Temperature stability of the skyrmionic bubbles in a 600 nm nanostripe at temperatures ranging from 373 to 630 K. (a−d) Under-focusedLTEM images of skyrmionic bubbles in the 600 nm nanostripe taken at temperatures ranging from 300 to 630 K under their corresponding criticalmagnetic field. All the photos are taken in the same region of the nanostripe. The scale bar is 500 nm. (e) Upper plane: schematic diagramillustrating the morphology of skyrmion bubbles and the distance between them. The shape of the bubble is assumed to be elliptic shape. The half-major axis, half-minor axis, and distance between the bubbles are represented by a, b, and d, respectively. Lower plane: temperature-dependent valuesof 2a, 2b, and d. The half-filled circles represent the average value; the error bars are added based on results summarized from parts a−d.

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TEM images of magnetic bubbles at different temperaturesunder their corresponding critical magnetic field (i.e., theapplied field needed for the formation of skyrmion bubbles).Intriguingly, we found that the single skyrmion bubble chaincould keep stable even up to an extremely high temperature of630 K. In comparison with the critical temperature observed inthe 4000 nm Fe3Sn2 nanostripe, the one in the 600 nmnanostripe is significantly enhanced by approximately 100 K.Our results thus strongly suggest that the width confinement isbeneficial for the thermal stability of skyrmion bubbles. Topresent the high thermal stability in a more quantitativemanner, as shown in the upper plane of Figure 4e, we describedthe ellipticity of the skyrmion bubble defining two parametersof the major semiaxis a and minor semiaxis b. By plotting thevalues of a and b as a function of temperature in the lowerplane, Figure 4e, we found that both values were temperature-independent, indicating that the size of the skyrmion bubblesremained almost a constant over the wide temperature range300−630 K. Moreover, the average distance d between twoneighboring skyrmion bubbles also kept unchanged within thiswide temperature range, further confirming the high stability ofthe single skyrmion bubble chain. The realization of the highlystable single skyrmion bubble chain in the geometricallyconfined Fe3Sn2 nanostripes can be attributed to (i) the weaktemperature-dependent magnetic anisotropy Ku of the Fe3Sn2crystal for which the value of Ku remains rather stable astemperature increases from 300 to 630 K (Figure S8, SI); and(ii) the formation of edge states at the boundaries of thenanostripes. For the frustrated magnets, edge states originatefrom a strong easy-plane surface anisotropy due to the lowersymmetry of magnetic irons at the boundaries.42 In theformation process of skyrmion bubbles, it can act as the initialnucleation centers, as is the case in the highly geometricallyconfined FeGe nanostripe.31,52 This feature not only helps theskyrmion bubbles form at a relatively low field (Figure S9, SI),but also enhances their temperature stability in the 600 nmnanostripe. We should note that the critical field to formskyrmion bubbles in the Fe3Sn2 nanostripe is larger than that inthe polar magnet GaV4S8 with strong anisotropy,7 magneticfilms with interfacial DMI interaction,16,52,53 and chiral magnetsFe0.5Co0.5Si

4 and Cu2OSeO3.6 However, very recently, both the

zero-field stable skyrmions and skyrmion bubbles have beenrealized via a simple field cooling manipulation in the Co−Mn−Zn21,22 and MnNiGa,23 respectively. It is hoped that wecan realize the single skyrmion bubble chain without anysupport of external magnetic fields in the Fe3Sn2 nanostripes inour future work.In conclusion, we have successfully created a highly stable

single chain of skyrmion bubbles in a geometrically confinedFe3Sn2 nanostripe. In contrast to other centrosymmetric ornoncentrosymmetric materials, the highest stable temperatureof the skyrmion bubbles in the 600 nm Fe3Sn2 nanostripe isrecord-high, as shown in Figure 5. This extreme stability can beattributed to the weak temperature-dependent magneticanisotropy and the formation of edge states at the boundariesof the nanostripes. The realization of a single chain of skyrmionbubble in the Fe3Sn2 nanostripe over an extremely widetemperature range well above room temperature is a veryimportant step toward the fabrication of skyrmion-basedracetrack memory devices.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.nano-lett.7b04900.

Fabrication and characterization details, X-ray diffractionpattern, SEM-EDX results, schematics, sample thicknessvs sample positions, under-focused LTEM images, high-resolution TEM images, simulation details, and magneticmeasurements (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: xixiang.zhang@kaust.edu.sa. Fax: +966-12-8020221.*E-mail: wenhong.wang@iphy.ac.cn. Fax: +86-010-82649485.ORCIDXi-xiang Zhang: 0000-0002-3478-6414Wenhong Wang: 0000-0002-0641-3792Author ContributionsZ.H., Q.Z., and G.X. contributed equally to this work.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the National Key R&D Program ofChina (Grant 2017FA0303202), National Natural ScienceFoundation of China (Grants 11604148, 1561145003,11574374), King Abdullah University of Science andTechnology (KAUST) Office of Sponsored Research (OSR)under Award CRF-2015-2549-CRG4, and Strategic PriorityResearch Program B of the Chinese Academy of Sciences underGrant XDB07010300.

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Figure 5. Comparison of the highest stable temperature of theskyrmions and skyrmion bubbles in noncentrosymmetric andcentrosymmetric materials, respectively. The data are taken from thepublished studies, including refs 3, 5, 29, 31, 35−37, and 54 based onthe in situ Lorentz TEM observations.

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Nano Letters Letter

DOI: 10.1021/acs.nanolett.7b04900Nano Lett. 2018, 18, 1274−1279

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